Telephone amplifiers have been used in the past supplementing normal telephone hand sets, which are not hand-held with such amplifiers. These prior art units may be classified in several groups:
(A) Amplifiers Only
These devices amplify both sides of a conversation over a common loud speaker. The handset is still hand held, and when the amplifier volume is high, feedback results in howls and squeals.
(B) Cradled Handset Amplifier
These instruments require the handset to be placed in a cradle so that the off-line voice is amplified. Some type of accoustical horn in generally necessary to direct the user's voice into the handset transmitter, or mouth piece. The loud speaker of the unit is generally separated from the cradle device and the speaker must be kept low enough to prevent feedback.
(C) Switching Amplifier
The switching amplifier generally contains its own microphone and speaker which are switched on and off alternately in response to voices present. That is, when the voice signals generated by the microphone stops or lowers considerably, then the loud speaker is turned on to hear the conversation from from the other party on the telephone line. This device is limited in the loudness of the loud speaker in order to prevent inadvertant switching, resulting in a breakup of the conversation.
In all of the described amplifiers, the inherent problems deal mostly with the accoustical feedback between the transmit and receive causing unwanted distortion and interruptions of the conversations. Attempts to overcome these defects have resulted mostly in multiple piece assemblies that physically separate the microphone and speaker in order to achieve the necessary accoustical separation and isolation. The results are both poor voice qualities and ungainly appearances. In most cases the party on the other end of the telephone line is greatly disturbed by the unnatural effects of the telephone amplifier.